Muscles:



Muscles:

Bones cannot move without muscles.

Cell contraction is the changing of the shape of the cell. If a cell contracts in one direction, it lengthens in another direction. Some cells are designed to contract; these are muscle cells, which are found in most animals. These types of movements rely on biochemical systems involving tubulin and microtubules. Another basic cell movement depends on the protein actin found in the eukaryotic system. Actin fibers cause movement in the cytoskeleton and cell membrane, using energy, which comes from ATP.

There are three types of muscles:

They are divided according to their microscopic structure and the nerves that activate them.

1) Smooth Muscle: smooth, glistening in appearance, also called involuntary muscle. Found in various internal organs such as, in the walls of blood vessels and the digestive tract.

2) Cardiac Muscle or Heart muscle: has control centers of its own.

3) Skeletal Muscle: striated or voluntary muscle.

These muscles are controlled by the somatic (voluntary) nervous system. They can contract much more rapidly than smooth muscles or cardiac muscle, but they can't stay contracted for long periods. Skeletal muscles are multinucleated; the nuclei lay just beneath the cell surface.

Muscles are an arrangement of connective tissue and muscles. Each muscle fiber contains a precise arrangement of protein. A whole bunch of these fibers is called FASCICULI (Fascicles) and gives meat its stringy appearance. A single muscle fiber is a muscle cell. Myofibrils make up a single muscle cell. A muscle cell contains a number of myofibrils, which is composed of a number of fused cells. During embryonic development, the myoblast (muscle stem cells) fuse to form a muscle fiber. The nuclei are left behind. Skeletal muscles are multinucleated. Some myoblasts don’t fuse and become satellite cells. These cells will become active during muscle injury where they help with muscle healing.

Blood vessels run throughout the fasciculi supplying oxygen and nutrients while getting rid of wastes and carbon dioxide. Nerves also penetrate the bundles and divide so that each fiber has a nerve. The whole mass of aligned fasciculi is the belly of the muscle. The belly is enclosed by FASCIA.

The fascia is connective tissue that is made up of dense collagen fibers, called the epimysium. There is a layer of fascia that separates each fasciculi called the perimysium. This is also made up of collagen with elastin. The perimysium also surrounds blood vessels. Each fasciculi has its own blood vessels and nerve.

Within the fasciculi, each myofibril (or muscle cell—fused myoblasts) is also surrounded by a thin layer of connective tissue called the endomysium. This thin layer of connective tissue contains capillaries, satellite cells and nerve fibers.

All the connective tissues: epimysium, perimysium, and endomysium come together at the end of each muscle and fuse. This bundle of connective tissue is called a tendon or aponeurosis (broad sheet of connective tissue). The tendon or aponeurosis will bind to a bone where the collagen fibers extend into the bone matrix. This is a firm attachment. When the muscle contracts, it gets shorter and pulls on the tendon, which pulls on the bone and you move. When muscles contract, one end moves (INSERTION) and one does not move much (ORIGIN). Bones are returned to their original position by opposing muscle actions.

Muscle Antagonism:

Two opposing muscle groups are called antagonistic muscles.

Example: Movement of lower arm.

Major muscles that flex and extend the lower arm originate on the humerus and scapula.

Flexor: biceps brachii- originates at two points, inserts on the radius.

Antagonist: Extensor: triceps brachii-- lies on the back of the arm. It originates at three places, two on humerus and one on the scapula.

Here are some voluntary muscles and their functions.

Frontalis: wrinkles the forehead and lifts the eyebrows.

Orbicularis oculi: closes your eye and allows you to wink.

Zygomaticus: raises the corner of your mouth and allows you to smile

Orbicularis oris: closes your lips so you can kiss.

Sternocleidomastoid: Turns your head side to side and flexes your head and neck.

Trapezius: shrugs your shoulders.

Latissimus dorsi: pulls your arms back.

Deltoids (anterior, medial, posterior): raises your arms

Rectus abdominis: allows you to sit up.

Pectoralis major: pulls your arms across your chest

External oblique: rotates trunk

Bicep Brachii: flexes the forearm.

Tricep Brachii: Extends the forearm.

Flexor carpi: flexes the hand

Extensor carpi: extends the hand

Flexor digitorum: flexes the fingers

Extensor digitorum: extends the fingers

Sartorius: crosses your legs

Quadricep femoris group (4 muscles): extends your lower leg

Hamstring group: flexes your lower leg

Gluteus maximus: extends the thigh

Gluteus medius (on hip): moves the leg out.

Adductor longus: pulls leg in.

Tibialis anterior: flexes foot

Gastrocnemius: extends foot

Flexor digitorum longus: flexes toes

Extensor digitorum longus: enxtends toes

Flexing requires the contraction of the biceps and the relaxation of the triceps. In relaxing the arm, the action reverses itself.

Ultrastructure of skeletal muscles:

The muscle fiber:

1) Each muscle fiber is surrounded by a cell membrane, the SARCOLEMMA.

2) The sarcolemma receives the endings of the motor neurons at the neuromuscular junction.

3) Motor neurons send impulses from the central nervous system to muscles, which cause the muscles to contract.

4) Just below the sarcolemma there are a number of mitochondria, nuclei, and glycogen granules, a sarcoplasmic reticulum and T-tubules, which spread the action potential to the sarcoplasmic reticulum.

5) Below these structures are rod-shaped myofibrils.

Let's look at one myofibril: we see banding, which is called a Z line.

Between the Z lines there is the contractile unit called the SARCOMERE.

Toward the center is a broad region called the A band with a smaller lighter region called the H zone. The A band is made up of overlapping myofilaments. Myofilaments are filamentous protein structures of actin and myosin. The dark lines extending across the A band and running through the H Zone are myosin filaments. The myosin filaments are overlapped by actin filaments which begin at the Z line and run part way through the A band. The I Bands consist of actin filaments alone.

When the muscle fiber is stimulated, the Z lines move together and sarcomere is shortens. The banding changes producing a dark line where the H zone was. Actin myofilaments slide inward through the myosin. The I bands become reduced.

How does this happen?

Myosin consists of a fibrous 'tail' with a globular 'head'. The tail is where the individual myosin molecules join to form a thicker filament. The myosin head binds to an ATP molecule (low energy state), which can be hydrolyzed to ADP and P. The energy released by the cleaving is transferred to the myosin and changes the shape of myosin to a high-energy configuration. The high-energy myosin binds to a specific site of actin and forms a cross-bridge. Once attached to actin, the stored energy is released, and the myosin head relaxes to its lower energy configuration. When it relaxes, it changes the angle of the attachment of the myosin head to the fibrous myosin tail. The myosin bends inward on itself and pulls the thin filament toward the center of the sarcomere. The bond between the lower energy myosin and actin is broken when a new molecule of ATP binds to the myosin head. The process repeats itself.

Each of the approximately 350 heads of the myosin filament. Join and rejoin about 5 cross bridges per second.

A muscle stores enough ATP for a few contractions. Although muscles store glycogen between the myofibrils, most of the energy needed for repetitive muscle contraction is stored in PHOSPHAGENS. These substances can supply a phosphate group to make ATP from ADP.

When at rest, the myosin binding sites on the actin molecules are blocked by TROPOMYOSIN, a regulatory protein. The TROPONIN COMPLEX, another set of regulatory proteins, positions the tropomyosin on the actin filament.

For a muscle cell to contract, the myosin-binding sites on the actin must be exposed. Calcium ions bind to the troponin complex, which changes the shape of the complex. This changes the interaction between the troponin complex and tropomyosin. This change exposes the myosin binding sites on the actin.

The calcium concentration in the cytoplasm of the muscle cell is regulated by the sarcoplasmic reticulum. The membrane of the sarcoplasmic reticulum actively transports calcium from the cytoplasm into the interior of the reticulum, which is an intracellular storehouse for calcium.

Steps for muscle contraction:

1) The motor cortex of your brain determines which muscles you will contract for the specific function, and sends an impulse down the correct nerve for that specific muscle.

2) An impulse moves down a nerve cell and through the motor neuron.

3) The motor neuron stimulates the SARCOLEMMA. The action potential spreads deep into the interior of the muscle cell along the infoldings of the T tubules.

4) The T tubules stimulate the SARCOPLASMIC RETICULUM, which then releases calcium.

5) The calcium floods the sarcomere and binds to the troponin complex. The interaction between the troponin and tropomyosin is affected. They myosin-binding sites on actin are now exposed.

6) ATP on the myosin head hydrolyzes. The energy is transferred to the myosin and changes the shape of the molecule. The energized myosin binds to a specific site on the actin and forms a cross-bridge.

7) The energy stored in myosin is released. Myosin relaxes to the lower energy shape. As it relaxes, the myosin bends on itself, pulling the actin filament towards the center of the sarcomere. The cross-bridge between the actin and myosin is now broken.

8) The calcium is actively transported back into the sarcoplasmic reticulum. The troponin complex retains its original shape and holds the tropomyosin in place to block the actin binding sites on the myosin.

9) A new ATP molecule binds to the myosin head.

10) This continues until the muscle has contracted.

11) The muscle returns back to its original position, when the antagonistic muscle contracts.

Rigor Mortis occurs when there is no more ATP available, and the calcium is not removed from the sarcomere. The muscle stays contracted. Rigor mortis ends because bacteria have begun to break down the muscle filaments.

Fast and slow muscles differ in the duration of twitches.

Slow Fibers have less sarcoplasmic reticula than fast fibers. Calcium remains in the cytoplasm longer. The twitch lasts up to 5 times longer than the fast fiber.

Slow twitch fibers have many mitochondria, a rich blood supply, and MYOGLOBIN (an oxygen storing protein). Myoglobin is a brownish-red pigment, which binds oxygen more tightly than hemoglobin. Myoglobin is found in the dark meat of poultry and fish.

Contractions:

There are two types of muscle contractions: Isotonic and Isometric.

Isotonic: The skeletal muscle shortens and pulls on the bone. The bone moves. Heavy weight lifting (and sudden bouts of exercise) can make your muscles sore. In doing these types of activities, you can rip myosin heads and actin filaments (as well as damage cells: mast cells and other tissue cells. The mast cells will release histamine, and damaged cells can release prostaglandins. Both of these compounds will cause an inflammatory response, the tissue will fill with fluid and swell. This slight swelling can cause pain). As the actin and myosin are damaged, the muscle cell will have to produce more proteins. The more proteins produced the thicker the filaments (the more heads on the myosin filament—the more cross bridges, the more weight can be moved), the thicker the muscle.

Isometric: the skeletal muscle contracts, but the bone doesn’t move. The individual muscle fibers shorten and the connective tissue shortens.

Cardiac Muscle:

Characteristics of Structure:

• Single centrally placed nucleus

• Dependent upon aerobic metabolism for energy. There are a large number of mitochondria associated with cardiac muscle. Myoglobin is also used.

• Certain specialized sites known as intercalated discs are present. The cell membranes of two adjacent cardiac cells are bonded together by desmosomes and gap junctions. The gap junctions allow ions and small molecules to move from one cell to another. This creates an electric current that can flow from cell to cell. The intercalated discs lock the two cells together. This allows them to be anchored, and they can pull together, making them more efficient.

• Cardiac muscles contract without neural stimulation. Each muscle has its own pacemaker. When two cells touch the contractions synchronize.

• Contractions last up to ten times longer than skeletal muscles.

Smooth Muscle:

Characteristics:

• Found: Integumentary System: around blood vessels which regulate blood flow. Also found around the hair to make them stand on end in piloerection. Cardiovascular System: surround blood vessels. These can contract and relax, which helps with controlling blood pressure. Respiratory System: the smooth muscles around the air passages can change the diameters of these tubes. Digestive System: the smooth muscles here move the food along. Excretory System: smooth muscles in the walls of the ureters and bladder move the urine along. Reproductive System: the smooth muscles in male’s causes ejaculation. In females, the smooth muscles move oocytes and the muscles in the uterus will push the babies out during birth.

Structure:

• Long and slender

• Spindle shaped with a centrally located nucleus.

• There are no T-tubules. The sarcoplasmic reticulum forms a loose network. There are no myofibrils and no sarcomeres. In fact, there are no striations.

• The myosin filaments are scattered throughout the sarcoplasm. Each filament has a high number of heads.

• The actin filaments are attached to dense bodies, which are made up out of desmin, a protein. Some dense bodies are attached to the sarcolemma (and anchored by intermediate filaments). When filaments slide, they pull on the dense bodies, which pulls on the sarcolemma.

Smooth muscles are surrounded by connective tissue—no tendons.

Osteoporosis and muscle contraction.

Postmenopausal women do not easily absorb dietary calcium, but they need calcium for muscle contractions. The calcium is removed from the bones. Bones become brittle over time and break easily. Females need to increase calcium intake when young to help prevent osteoporosis.

Talk about: sprains, strains tetanus, ligament damage…

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